Conventionally, there has been known a sensor that has the characteristic of allowing a current corresponding to an oxygen amount to flow therethrough by the application of a DC voltage and is capable of detecting the oxygen amount on the basis of a value of the output current such as an oxygen content sensor or an air-fuel ratio sensor. Generally, such a sensor has a large temperature dependence, and output thereof thus varies depending on an element temperature. Thus, it is necessary to maintain the element temperature at an appropriate temperature (active temperature) in order to maintain an excellent accuracy of oxygen amount detection. Thus, there is a system that includes a heater attached to a sensor, and feedback-controls energization to the heater so as to maintain the element temperature at an active temperature by heat generation of the heater (refer to Patent Literature 1, for example).
In this system, it is necessary to detect the element temperature to feedback-control the energization to the heater. However, disposing a temperature sensor which directly measures the element temperature on the above sensor may result in upsizing of the sensor and a cost increase. In view of this, there has been proposed a method that estimates an element temperature of a sensor on the basis of information other than the element temperature. For example, Patent Literature 1 describes a method that detects a sensor AC current ΔI generated from an AC voltage ΔV applied to an oxygen content sensor, calculates an AC impedance Z(=ΔV/ΔI) or an admittance Y(=1/Z=ΔI/ΔV) of the sensor, and estimates an element temperature of the oxygen content sensor on the basis of the impedance Z or the admittance Y. Patent Literature 1 also describes switching performed in such a manner that the impedance Z is calculated to use the temperature estimation in a low temperature region, and the admittance Y is calculated to use the temperature estimation in a high temperature region, taking the temperature characteristics of the impedance Z and the admittance Y into consideration.
However, the conventional method for estimating an element temperature as described in Patent Literature 1 is susceptible to further improvement for estimating an element temperature of a sensor with high accuracy.
Patent Literature 1: JP-2000-65781-A